By microinjecting substances through hollow recording electrodes, the effect of the injected substance upon the activity of neurons immediately below the electrodes may be examined. This technique can be used with the freely moving animal, allowing simultaneous measurement of neural activty and behavior. Pilot work by the author has indicated that morphine injected into periacqueductal gray (PAG) of the rat brain produces analgesia and an increase in neural activity at the site of injection. These effects of morphine microinjection do not occur at other sites. The proposed research will examine the effect of repeated morphine injection into the PAG and other sites to examine the neural and behavioral correlates of tolerance and physical dependence. Further, the interaction between morphine and a morphine auntagonist will be examined by simultaneously monitoring neural activity and behavior follwoing their direct application to a circumscribed brain region. the method of electrical recording during microinjection of substances into the brain will then be ued to investigate the involvement of different neurotransmitter systems in morphine analgesia.